replace %svelte.body% etc with %sveltekit.body%

[Rich-Harris]

Never got round to creating an issue for this, but I think it's a no-brainer — we should have been more explicit from the start that %namespace.body% is a SvelteKit thing, not a Svelte thing

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[benmccann]

I saw this the other day and was also thinking it's inconsistent with what we've done elsewhere

[elliott-with-the-longest-name-on-github]

@Rich-Harris

While we're at it, the root context for $app/stores is called __svelte__ (see here). Seems like renaming it to __sveltekit__ would fit well with this PR?

[Rich-Harris]

We can make the __svelte__ change separately since it's internal rather than external — it would probably be better to use a private object as the key rather than a magic string, but I don't know off the top of my head if that'd be a rabbit hole